You're making the assumption that the jobs require exerting oneself to the peak of human physical capability. If that were so, then yes, a man is going to outperform a woman — I don't dispute that men are just biologically advantaged in terms of physical strength. But most blue collar work doesn't require the kind of physical prowess that only a man can reach at the extremes of our biology. From my own experience with farm and dairy work, the vast majority of tasks and labor are well within the capabilities of women regardless of biology.